2. Such terms as atomic weight, atom, and molecule are now employed in a stricter sense than formerly. Every element has been held from the time of Dalton to have a number called its atomic weight. This number, according to Dr. Frankland, one of our most distinguished modern chemists, is made to represent, as far as possible: lst, The smallest proportion by weight in which the element enters into or is expelled from a chemical compound—the smallest weight of hydrogen so entering or leaving a chemical compound being taken as unity. 2d, The weight of the element in the solid condition at any given temperature contains the same amount of heat as seven parts by weight of solid lithium at the same temperature. 3d, The weight of the element which, in the form of gas or vapor, occupies, under like conditions of temperature and pressure, the same volume as one part by weight of hydrogen."—Lecture Fotes for Chemical Stu dents, 1866, p. 2. Recent investigations have led chemists to assign to many of the elements double the atomic weights that were previously assigned to them taking as formerly the atomic weight of hydrogen as the unit, the atomic weight, or, as it is now often styled, the atomic number of oxygen is changed from 8 to 16, that of carbon from 6 to 12, that of sulphur from 16 to 32; and this doubling is by the latest writers extended to most of the elements except the halogens, nitrogen, phos phorns, boron, the metals of the alkalies, gold, and silver. The old atomic weights are still recognized as combining or equivalent numbers. The reason why this doubling of the number has been adopted will be presently given. The distinction between an -atom and a molecule must be clearly recognized. "We may define an atom of an ele mentary body to be the smallest proportional weight thereof which is capable of existing in chemical combination.; and we may define the molecule of an elementary body to be the smallest proportional weight thereof which is capable of existing in the free or uncombined state." This, which is Hofman's definition (Modern Chemistry, p. 157), is now accepted. Thus a molecule (or elementary molecule, as it is often termed) may consist either of an isolated atom, or of a group of atoms.
The bulk of a molecule, or the molecular volume of an element in the gaseous or vaporous state, is the same as the molecular volume of hydrogen at the same temper ature and pressure, and in a large number of cases the molecular weight of an element is twice its atomic weight. Dr. Frankland gives the following list of the elements whose molecular volumes have as yet been determined: The molecules of mercury, cadmium, and zinc contain one atom, and are termed monatomic molecules; those of hydrogen, oxygen, chlorine, bromine, iodine, fluorine, nitrogen, sulphur, and selenium. contain two atoms, and are termed diatomic molecules; the molecules of oxygen, as ozone, contain three atoms, or are triatomic; while those of phosphorus and arsenic are tetra, Comic, and those of sulphur under certain conditions are hexatomic. Thus an element, as in the cases of oxygen and sulphur, may, under' different conditions, have two dis tinct molecular weights.
3. We shall now proceed to explain the reasons why many of the atomic weights have been doubled. "It is obvious," says Dr. Odling, in his elaborate article on "Atomic Weights" in Watts's Dictionary of Chemistry, vol. i. p. 456, "that the atomic weights of an element and of its combinations should be selected so as to express the entire series of combinations by the simplest series of formula:; so as best to accord with the chemical properties and metamorphoses of the bodies; so as best to illustrate their analogies with other bodies; and so as to be in relation with their physical prop erties, such as their specific volumes, specific heats, isomorphism, etc." We shall endeavor to show how he applies these views to prove that, in the case of oxygen, 16 parts of that element, or the quantity thereof which unites with 2 atoms of hydrogen, is the smallest proportion of oxygen that can enter into a combination. "We find, in the first place," says Dr, Odling, " that the quantity of oxygen contained in the great majority of definite oxidized compounds must necessarily be represented by 16, or some multiple of 16 parts. Thus, the molecules of all hydrates, double oxides, acids, oxisalts, aldehydes, ketones, alcohols, oxacid-ethers, and a great number and variety of - other .compounds, doubtless forming together 99 per cent of all known compounds of oxygen, cannot be represented save with 16 parts, or some multiple of 16 parts of oxygen. For example, the molecules of hydrate of potassium.* benzoic aldehyde, acetone, chloral, hypochlorite of sodium,* etc., each contain 16 parts of oxygen. The molecules of spinelle, brown &motile, camphor, benzile, acetate of sodium,* benzoic acid, etc. each contain twice 16 parts of oxygen. The molecules of nitric acid, gly
serin, chlorate of potassium,* salicylic acid, augite, etc., each contain three times 16 parts of oxygen. We need not carry the quotation further, it being sufficient to remark that Dr. Odling gives similar lists of substances whose molecules each contain 4, 5, 6, 7, etc., times 16 parts of oxygen. Hence it follows that when two bodies only differ in composition by. the different proportions of oxygen which they contain, that difference amounts to 16 parts, or some multiple of 16 parts of oxygen. This is well shown in the two following series of bodies given by Odling, in the former of which the symbols are arranged according to modern views: KCl, Chloride of potassium. 04114, Ethylene.
Hypochlorite of potash. 0411402, Aldehyde.
Chlorite of potash. Acetic acid.
Chlorate of potash. Glycolic acid.
Perchlorate of potash. Glyoxylic acid.
It is obvious that in both these series each term differs from the preceding one simply by 0,, or 16 parts of oxygen. Again, the quantity of oxygen which can be lib erated by any reaction, and which, either alone or in the form of water, can be added to or separated from a compound, must be 16, or some multiple of 16 parts. Thus, each molecule of nitrate of soda (NaO,NO,), when decomposed by heat, yields nitrate of soda (NaO,NO,) and (or 16 parts of oxygen); similarly, each molecule of per manganate of potash, when decomposed by sulphuric acid, yields manganese—alum, and (or twice 16 parts of oxygen); and each molecule of chlorate of potash (R is decomposed by heat into chloride of potassium (KOl) and 0, (or three times 16 parts of oxygen). Again, water (and consequently its main constituent. oxy gen) is always eliminated in double or some higher even atoms. Thus, formic acid yields carbonic oxide (0202) and two atoms of water (11202); alcohol yields olefiant gas and two atoms of water oxalate of ammo nia (NH2,021104) yields cyanogen (C2N) and four atoms of water (11404); and innu merable additional examples might be given. On these grounds (and many additional ones might be adduced if space permitted) it becomes obvious that if the vast majority of oxidized bodies were correctly formulated, they would be represented more simply by the formula in which 0 = 16 than by the formula in which 0 = 8. Reasons of a similar nature have led to the duplication of the atomic weight of carbon, sulphur, and many of the other elements. There must obviously be some means of distinguishing when 0 indicates 8 or 16 parts of oxygen, C indicates 6 or 12 parts of carbon, etc. Various modes of distinction have been adopted by different chemists. In Watts's Dictionary of Chemistry (published between 1863 and 1868), the new atomic weights are represented by the same symbols which have hitherto been adopted for the old weights; while the latter (when they are occasionally introduced) are printed in italic capitals; thus water is represented by H,0 in the new and by /10 in the old system, acetic acid by in the new and by in the old system, etc. A more common means of indicating when the value of the symbol of an element is doubled in value is by drawing a horizontal bar through it, a notation due to Berze]ius; thus, e,O,S• repre sent respectively an atom of carbon, of oxygen, and of sulphur in the new system. This system is useful in forming, as it were, a bridge to facilitate the passage from the old to the new system, and will gradually disappear when all chemists recognize the doubled atomic weights. Naquet, Miller (in the 3d edition of his Chemistry, 1864), and others, adopt this barred system, and the latter frequently gives the formula per taining to both systems; for example, or Pe- SO, IL2,6IL6, repre sents the composition of the crystallized sulphate of protoxide of iron often described as protosulphate of iron.—Inorganic Chemistry, 3d ed., p. 6. Some writers, as Frank land, in his Lecture Notes for Chemical Students, 1866, following the plan of Watts and the contributors to his dictionary, unreservedly adopt the doubled atomic Weights, and represent them by the old formula; thus 0, C, and S represent in these works precisely double the weight of oxygen, carbon, and sulphur that these capitals represent in the 1st and 2d editions of Miller's Chemistry, Fownes's _Manual of Chemistry, and other standard works published a few years ago. It is now customary for the writers of chemical papers who object to the barred symbols as being unseemly, to insert at the commencement C = 6, 0 = 8, or C = 12, 0 = 16, in order that the reader may be able to recognize which system is adopted.